1. Field of the Invention
The present invention relates to a power supply device, and in particular, to an AC (Alternating Current)/DC (Direct Current) power supply circuit for supplying both AC and DC voltages.
2. Description of the Related Art
A power supply device is provided for an electronic product such as, for example, a camcorder, a VCR (Video Cassette tape Recorder), or a TVCR which is a combined to product of a TV and a VCR.
FIG. 1 is a schematic diagram of a conventional AC/DC power supply circuit in application to a TVCR product model TVCR9D1 of Funai Electric Co. Japan. Referring to FIG. 1, a voltage generated from an AC power source 2 is transformed in a transformer 4, rectified in a diode 6, and smoothed in a capacitor 8. Hence, the AC voltage is converted to an about 12V DC voltage. The 12V DC voltage is supplied to various ICs (Integrated Circuits), a tuner, a motor, a microcomputer, a flyback transformer (FBT) 16 serving as a high voltage generator in a TVCR. When the DC voltage is supplied to the microcomputer, it passes through a regulator 14. In FIG. 1, a DC power supply unit 10 for directly supplying the 12V DC voltage is included in the AC/DC power supply circuit to render a corresponding product portable. A 12V DC power source 11 in the DC power supply unit 10 is supplied to the ICs, tuner, motor, microcomputer, and the FBT 16 in the TVCR, via a diode 12.
The FBT 16, generally operated at 125V, is redesigned to operate at 12V in the AC/DC power supply circuit of FIG. 1.
The AC/DC power supply circuit of FIG. 1 requires a transformer of a very high capacity, and generates much heat due to transformation from a high voltage to a low voltage. Further, the FBT 16 requires redesign depending on the size of a TVCR monitor, and a VCR cannot operate with a TV turned off because a secondary power supply of the FBT 16 is used as a VCR power supply. Therefore, for programmed recording requiring no display on a TV monitor, the VCR operates with the TV monitor muted. Here, muting the TV monitor refers to turning off the TV monitor while an operational power voltage is still supplied to the TV. Thus, unnecessary power is supplied to the other circuits and drivers except for TV monitor-related circuits and drivers, thereby increasing power consumption.